(1) Field of the Invention
This invention relates to a radiation signal-processing unit for correcting detection signals of annihilation radiation-pairs, and a radiation detector provided therewith. More particularly, this invention is directed to a radiation signal-processing unit that allows correction of a detection position of radiation, and a radiation detector provided therewith.
(2) Description of the Related Art
Description will be given of a specific construction of a conventional positron emission tomography (PET) device for imaging distribution on radiopharmaceutical. The conventional PET device includes a radiation ring having radiation detectors arranged circularly for detecting radiation. The detector ring detects a pair of radiation (an annihilation radiation-pair) having opposite directions to each other that is emitted from inside of a subject.
Next, description will be given of a construction of the radiation detector 51. As shown in FIG. 9, the radiation detector 51 includes a scintillator 52 having scintillation counter crystals arranged three-dimensionally, and a light detector 53 for detecting fluorescence from gamma rays absorbed into the scintillator 52. The radiation detector 53 has a detection surface where detecting elements are arranged in a matrix. The detection surface of the light detector 53 is optically connected with one surface of the scintillator 52. See Japanese Patent Publication No. 2004-279057.
Radiation entering into the scintillator 52 is converted into many photons to travel toward the light detector 53. Here, the photons, entering into the scintillator 52 while spatially spreading, enter into each detection surface of the light detector 53 arranged in a matrix. That is, many photons from fluorescence are simultaneously split into detecting elements for detection.
The radiation detector 51 determines a position in the scintillator 2 where fluorescence is emitted through detection data on fluorescence that is captured by two or more detecting elements. That is, the radiation detector 51 determines a position of a center of gravity in a luminous flux of fluorescence in the detection surface by two or more detecting elements. The position of the center of gravity means a position where fluorescence has been generated. Information on the position is used upon mapping of radiopharmaceutical within the subject.
However, the conventional detection of radiation noted above has a following drawback. Specifically, more doses of radiation entering into the radiation detector 51 may lead to incorrect identification of a position where fluorescence is generated.
This drawback concerns to a calculation process of the center of gravity in the luminous flux of fluorescence. Here, the calculation process is to be described. For simplification, it is assumed that the detection surface of the radiation detector 53 has 2 by 2 detecting elements, as shown in FIG. 10. The detection signals of fluorescence outputted from the detecting elements a1 to a4 are assumed to be A1 to A4, respectively. A1 to A4 represent fluorescence intensity detected by the detecting elements a1 to a4, respectively. A position X of the center of gravity in the luminous flux of fluorescence in an x-direction is expressed as follows under assumption of a center position as a starting point:X={(A1+A3)−(A2+A4)}/{(A1+A2+A3+A4)}  (1)
Here, letting (A1+A3) be Xa, and (A2+A4) be Xb, a relation of X=(Xa−Xb)/(Xa+Xb) holds.
Likewise, a position Y of the center of gravity in the luminous flux of fluorescence flux in a y-direction is expressed as follows under assumption of a position a5 as a starting point:Y={(A1+A2)−(A3+A4)}/{(A1+A2+A3+A4)}  (2)
Here, letting (A1+A2) be Ya, and (A3+A4) be Yb, a relation of Y=(Ya−Yb)/(Ya+Yb) holds.
Specifically, more doses of radiation entering into the radiation detector 51 may lead to a phenomenon of apparently increased fluorescence detection intensity. Next, description will be given of this phenomenon. FIG. 11 shows temporal variations in fluorescence that the detecting elements detect. Fluorescence emitted in the scintillator continues to be applied to the detecting elements for a while, although it is weak. Upon detecting of radiation, it takes much time to detect fluorescence in consideration of such afterglow disappearance. Consequently, the radiation detector 51 determines fluorescence taking no account of the afterglow. Specifically, as shown in FIG. 11, the radiation detector 51 integrates detection intensity outputted by the detecting elements a1 to a4 during a period P with a time for calculation of fluorescence detection intensity A1 to A4. Here, the afterglow is not considered as the fluorescence detection intensity.
More doses of radiation enter into the radiation detector 51, and subsequent fluorescence is emitted before afterglow of previous fluorescence disappears. That is, fluorescence having a temporal width will temporally overlap each other. Specifically, as shown in FIG. 12, an afterglow component illustrated by S is to be added in calculation of fluorescence detection intensity.
Such phenomenon occurs in every detection intensity of A1 to A4. Here, letting afterglow components concerning Xa, Xb, Ya, and Yb be expressed with α, β, γ, δ, respectively, the positions X and Y as the center of gravity calculated under existence of the afterglow components are as follows:X={(Xa+α)−(Xb+β)}/{(Xa+α)+(Xb+β)}  (3)Y={(Ya+γ)−(Yb+δ)}/{{(Ya+γ)+(Yb+δ)}  (4)
The afterglow components α, β, δ, γ have an approximately equal value. Consequently, the afterglow components in numerator of Equations 3 and 4 are offset. On the other hand, the afterglow components in denominator of Equations 3 and 4 are not eliminated, but rather added to increase. Accordingly, the positions X, Y have a value different from an actual value under influence of the afterglow components. Specifically, existence of the afterglow components may lead to increased denominator of Equations 3 and 4, thereby decreasing an absolute value of the positions X and Y.
Description will be given of influences that the afterglow components exert on mapping positions of the center of gravity. Now it is assumed that fluorescence is emitted from each center of the scintillation counter crystals that constitute the scintillator 2. Here, a point p in FIG. 13 is a fluorescence generating position. The radiation detector 51 identifies the fluorescence generating position, as shown in FIG. 13, under no influence of the afterglow components.
Where fluorescence to be detected includes the afterglow component, the radiation detector 51 cannot correctly identify the fluorescence generating position shown in FIG. 13. That is, the absolute value of X and Y in Equations 3 and 4 will be increased apparently under the influence of the afterglow. Accordingly, as shown in FIG. 14, the fluorescence generating position to be calculated deviate apparently toward a center of the scintillator 2, which reduces distribution in fluorescence generation. As above, according to the conventional art, the afterglow of fluorescence causes incorrect identification of the fluorescence generating position.